The present invention relates to optical elements operating in transmission in the infrared spectrum, and the subject of the invention is the creation of optical elements of this type, the main characteristic of which lies in the fact that they are provided with a high-density-polyethylene coating layer.
High-density polyethylene (HDPE) is a polymeric material which presents high transmittance of infrared radiation (approximately 80% for a thickness of 0.5 mm). The index of refraction is low, namely, approximately 1.5 both in the LWIR range (from 8 to 14 micron) and in the MWIR range (from 3 to 5 micron). It moreover presents high impermeability.
HDPE is therefore suited for the production of optical elements for focusing IR radiation, provided that the thickness of the material is kept small (less than 1 mm). For this reason, HDPE is commonly used for the production of Fresnel lenses or refractive microlenses having a very small diameter (and hence a small depth). It is not, instead, conceivable to produce HDPE refractive lenses of greater thickness on account of absorption.
As already specified above, the subject of the present invention is represented by the use of HDPE not as a basic material for IR optical elements, but as a coating for IR optical elements made of some other material (for example, hygroscopic salt, semiconductor materials, fluorides, chalcogenides, silver halides, and others), exploiting the unique properties of HDPE, at the same time keeping the thickness reduced.